Fermented milk, especially yoghurt, is a fermented milk product highly appreciated by consumers. The reason of being welcomed owes to that it comprises viable lactobacillus which are helpful to human health. It is generally recognized that per ml (or g) of product should comprise at least 105-107 cfu of viable lactobacillus, and at least 100 ml (or g) of product should be consumed every day so as to be helpful to human health. Therefore, it is very important to keep this lactobacillus viable in the fermented products before consumption.
However, under the conditions that cold chain is not perfect and the products are stored without refrigeration for relatively long time, the viability of lactobacillus in fermented milk is usually very low, even no viable bacteria can be detected (W. T. Hamann and E. H. Marth, Survival of Streptococcus thermophilus and Lactobacillus bulgaricus in commercial and experimental yoghurts, J. Food Protection, 47:781-786, 1984). The reason for this phenomenon is that storage under non-cold chain conditions leads to serious post acidification of yoghurt products, and over post acidification can speed up the death of lactobacillus. This point has been illustrated sufficiently in previous research reports (GUO, Qinquan and ZHANG, Lanwei, The main starter bacteria to induce the post acidification of yoghurt and the characteristics thereof, FOOD AND MACHINERY, 87 (1):14-16, 2002; W. T. Hamann and E. H. Marth, J. Food Protection, 47:781-786, 1984). Many developing countries lack perfect cold chain system, so fermented milk is often transported, stored and sold in a non-refrigeration state, which leads to degradation of product's quality and great shortening of product's shelf life.
Some documents have reported processes for increasing the cell count of viable lactobacillus. These processes include: adding antioxidant (such as VC), adding prebiotics (such as oligosaccharides), encapsulating lactobacillus into microcapsule, non-adding Lactobacillus bulgaricus and decreasing oxidation-reduction potential (such as adding cysteine) etc. . . . (N P. Shah, Probiotic bacteria: Selective enumeration and survival in dairy foods, J. Dairy Sci., 83: 894-907, 2000; A. Lourens and B. C. Viljoen, Int. Dairy J., 11: 1-17, 2001). However, the effect of these processes is rather limited.
The root cause of the problem lies in that most of lactobacillus can use lactose for fermentation, and lactose fermentation will produce lactic acid, thereby decrease the pH value of milk quickly, which directly leads to the decreasing of the cell count of lactobacillus. For example, when Streptococcus thermophilus and/or Lactobacillus bulgaricus are used as fermentation strains and the milk is fermented at about 43° C., big quantity of lactic acid will be produced in 2-5 hours, which makes the pH value of milk decrease quickly to below 4.5 and leads to the degradation of viability or death of lactobacillus. 
Thus, controlling acid production of fermented milk is the key to solve the problem that store fermented milk at ambient temperature with high viable cell count.
Lactobacillus rhamnosus (available from American Type Culture Collection, bacterial strain number: ATCC 53103) is a probiotic strain isolated from healthy human body (U.S. Pat. No. 4,839,281; U.S. Pat. No. 5,032,399), and it can get rid of endotoxin from blood plasma (U.S. Pat. No. 5,413,785). The distinctive characteristic of this bacterium is that it does not ferment lactose and is resistant to acid and oxygen. Since this bacterium has the function of prevention and treatment of stomach upset (such as diarrhea) and improvement of immunity, it is used in healthy and functional products (such as yoghourt). The details of the biological characteristics and probiotic functions of this bacterium, please refer to above-mentioned patents and the review article—M. Saxelin, Lactobacillus GG A human probiotic strain with thorough clinical documentation, Food Rev. Int., 13: 293-313, 1997.